This invention relates generally to digital-to-analog converters and, more particularly, to high-performance superconducting digital-to-analog converters.
High-performance superconducting digital-to-analog (D/A) converters are required in a variety of applications for high-speed, low-power D/A conversion. One such application is digital superconducting flux-locked magnetometers, which provide highly sensitive detection of very small magnetic fields. Digital superconducting magnetometers have been proposed for use in such diverse areas as oil and mineral exploration, submarine and mine detection, and noninvasive medical diagnostic procedures.
A superconducting magnetometer typically employs one or more superconducting quantum interference devices (SQUID's) for measuring the strength of a magnetic field. A SQUID is a superconducting loop having a Josephson tunnel junction as a weak link in the loop. The weak link functions as a flux gate to allow the amount of magnetic flux in the loop to change. The amount of magnetic flux in the superconducting loop is quantized, which means that the loop supports only integer multiples of a flux quantum. When a SQUID is placed in a magnetic field, a superconducting current flows around the loop to oppose any change in the magnetic flux. However, when the opposing current exceeds the critical current of the superconducting loop, the Josephson junction opens the loop and allows one or more flux quantum to enter.
A superconducting magnetometer is constructed by adding a feedback path to a SQUID, thus allowing a current to be applied to the superconducting loop that cancels the current induced by the magnetic field. This maintains the magnetic flux in the loop constant, while the amount of current applied to the loop to balance out the induced current is a measure of the magnetic field. In a digital superconducting magnetometer, a superconducting D/A converter is required in the feedback path to convert the digital feedback signal to an analog current.
Conventional superconducting D/A converters typically employ a binary network of latching Josephson junctions and resistors which generates a set of reference voltages based on the current-voltage characteristics of the junctions. The voltage of a Josephson junction is zero in the superconducting or zero voltage state and equal to the energy gap voltage of the superconducting material in the nonsuperconducting or voltage state. However, using the energy gap voltage as the reference voltage requires uniform Josephson junctions having steep conductances at the gap voltage. The uniformity in the energy gap voltage must be 1/2 bit greater than 2.sup.N, which for 10 bits of resolution (N), requires less than a 5.times.10.sup.-4 variation in the gap voltages of the Josephson junctions, or about one microvolt. The latching nature of conventional superconducting D/A converters also requires synchronous clocking to reset the junctions during each clock cycle, resulting in high power dissipation. Accordingly, there has been a need for a superconducting D/A converter that does not have these drawbacks. The present invention clearly fulfills this need.